Internal combustion engines can be classified as segregating or non-segregating engines. All such engines use a compression stroke which precedes the ignition and combustion of fuel which is mixed with air.
In a non-segregating engine the fuel is mixed with the air before the start of the compression stroke as is the case with spark ignition gasoline engines commonly called SIGE engines. In some SIGE engines, known as stratified charge engines, which are not now common, the fuel is introduced to the air during the compression stroke but well before ignition which is started with a spark. In all non segregating engines the maximum compression pressure is limited since a pre-mixed air-fuel gas mixture can be ignited by the high temperature generated in the compression process before the spark occurs.
SIGE engines require the air fuel mixture to be nearly chemically correct. This restriction together with lower compression ratios and the need to throttle the air input at part load, which are all associated with this combustion system, result in a relatively poor thermal efficiency for the SIGE engine. Its main advantage is a fast combustion process, hence higher engine speed and power brought about by the fast burn of a pre-mixed gaseous mixture of fuel and air.
A segregating engine will compress all, or most of the air, without fuel and introduce the fuel into the air near the end of the compression stroke at the point where ignition is to be started. The familiar segregating engine is the diesel engine which injects liquid fuel into the combustion chamber, under very high pressure, near the end of the compression stroke.
The segregating engine has the capability of much higher thermal efficiency compared with the SIGE engine, particularly at part load. Its compression pressures, which enhance efficiency, are not limited by the danger of pre-ignition. No throttling is required at part load so avoiding pumping losses. Lean burning at part load is possible which also improves thermal efficiency.
The disadvantage of the diesel segregation method is the relatively long time it takes to inject the liquid fuel and vaporise it before it can ignite and burn fast. The diesel engine is therefore thermally more efficient than the SIGE engine but cannot run at the same high RPM as the SIGE and produces less power from a given size and weight. At high loads and high speed, combustion proceeds well into the expansion stroke, greatly penalising the thermal efficiency of the diesel engine.